Lead levels in some water samples across the country are 15 times over the legal limit.
Unsafe levels have been found in drinking water samples in more than 30 areas since 2017.
Irish Water figures obtained by the Irish Times show that a sample near Sutton Dart Station in Dublin was found to be the most contaminated by lead.
There were also unsafe levels in Kilkeedy in Clare, with a sample last year found to be 10 times over the legal limit for safe consumption.
Meanwhile, a test carried out in Sligo was found to contain 6 times the legal limit.
Unsafe levels were also identified in Kilmacthomas in Co Waterford, Gorey in Wexford and Screggan in Offaly.
According to HSE guidelines, long-term exposure to lead can cause high blood pressure, and potential harm to kidneys.
It could also affect brain development in children or to babies in the womb.
Irish Water says there were no lead water mains in Ireland, however there are still some lead pipes.
It added that its planning to remove all remaining lead pipes from the public water network and replacing them with plastic ones.